The present invention relates to an arrangement for measuring the position of a feed beam relative to a cradle and/or measuring the position of a rock drill relative to the feed beam in a drill unit in a rock drilling apparatus, comprising a cradle attachable to a boom, a feed beam installed in the cradle movably relative to the cradle in the longitudinal direction of the feed beam, and a rock drill connected to the feed beam movably in the longitudinal direction of the feed beam, the arrangement comprising an elongated toothed indicator member fixed to the feed beam in its longitudinal direction and a gear connected to the indicator member and contacting the toothing of the indicator member, wherein with the movement of the feed beam relative to the cradle or respectively the movement of the rock drill relative to the feed beam the indicator member rotates the gear and the indicator measuring the rotation of the gear emits signals proportional to the movement.
When a hole is being drilled with a rock drilling apparatus, it is important to know how deep a hole has been drilled. To determine the accurate depth of the hole, the position of the feed beam relative to the cradle of the rock drill must also be known. In known equipment, a rack secured to the feed beam in the longitudinal direction thereof is employed to indicate the position, said rack rotating a pinion contacting it and thereby a resolver connected to the pinion. The resolver in turn transforms the direction and length of the movement to an electrical signal that can be employed for example for controlling automatic control devices.
The problem with the known solution is that the pinion and rack are subject to wear on the action of their relative movement, particularly when dirt and rock dust enter between them. Likewise, the teeth of the rack are easily damaged when they hit rocks, and consequently can also damage the pinion. On account of shocks and vibration, the pinion can jump over a tooth in the rack, which causes a measuring error and thereby also errors in drilling. Further, vibration and shocks can result in tearing of the fastening sleeves of the pinion, and thus the pinion can move so far from the rack that it no longer properly contacts it, with the result that measuring is practically impossible. The fixing of the rack to the feed beam also involves extra manufacturing steps and a considerable amount of work in installation and replacement, since the rack must be fixed to the feed beam with screws or similar at suitable intervals.
On account of the above factors, for example the zero point of the sensor may shift, and thus for example in automatic drilling the sensor will give incorrect values. As a result, the moving of the feed beam to the vicinity of the rock surface to be drilled, i.e. driving into rock, is hindered and the feed beam can hit the rock surface at too high a speed or with too high a force and consequently be damaged. Similarly, with automatic control the drill boom can behave differently than expected on account of incorrect values. Therefore, it is necessary in all rock drilling machines to calibrate the sensor measuring the position of the feed beam often, which causes extra service and repair work.
Furthermore, Swedish Published Specification No. 453 111 discloses a solution in which the longitudinal movement of the feed beam is measured by employing the quantity of the hydraulic flow of the feed motor of the rock drill. The drawback of this solution is that The clearance and wear of the motor cause a measuring error, and furthermore said error varies with the variation of the transfer resistance. The measuring value thus obtained is not sufficiently reliable in the long run, and also requires frequent calibration.